Fully-depleted silicon-on-insulator transistors

1. A fully-depleted silicon-on-insulator (FDSOI) semiconductor structure, comprising: a first PFET, a second PFET, and a third PFET each having a different threshold voltage and each being over an n-well that is biased to a first voltage; and a first NFET, a second NFET, and a third NFET each having a different threshold voltage and each being over a p-type substrate that is biased to a second voltage, wherein the second voltage is different than the first voltage, the first voltage is a power supply voltage (VDD); and the second voltage is ground.

2. The structure of claim 1 , wherein: the first PFET is a high threshold voltage PFET; the second PFET is a regular threshold voltage PFET; the third PFET is a low threshold voltage PFET; the first NFET is a high threshold voltage NFET; the second NFET is a regular threshold voltage NFET; and the third NFET is a low threshold voltage NFET.

3. The structure of claim 2 , wherein: a threshold voltage of the first PFET is a predetermined amount higher than a threshold voltage of the second PFET; the threshold voltage of the second PFET is the predetermined amount higher than a threshold voltage of the third PFET; a threshold voltage of the first NFET is the predetermined amount higher than a threshold voltage of the second NFET; and the threshold voltage of the second NFET is the predetermined amount higher than a threshold voltage of the third NFET.

4. The structure of claim 1 , wherein: a transistor body of the first PFET and a transistor body of the second PFET each has a first dopant concentration; a transistor body of the third PFET has a second dopant concentration that is less than the first dopant concentration; a transistor body of the first NFET and a transistor body of the second NFET each has a third dopant concentration; and a transistor body of the third NFET has a fourth dopant concentration that is less than the third dopant concentration.

5. The structure of claim 4 , wherein: the second PFET is over a p-type back gate; and the second NFET is over an n-type back gate.

6. The structure of claim 5 , wherein: the p-type back gate causes a threshold voltage of the second PFET to be less than a threshold voltage of the first PFET; and the n-type back gate causes a threshold voltage of the second NFET to be less than a threshold voltage of the first NFET.

7. The structure of claim 5 , wherein: the second dopant concentration being less than the first dopant concentration causes a threshold voltage of the third PFET to be less than a threshold voltage of the second PFET; and the fourth dopant concentration being less than the third dopant concentration causes a threshold voltage of the third NFET to be less than a threshold voltage of the second NFET.

8. The structure of claim 1 , wherein the n-well is a single n-well in the substrate and is biased using a single n-well contact opening.

9. The structure of claim 8 , further comprising a buried oxide (BOX) layer on the substrate, wherein respective transistor bodies of the first PFET, the second PFET, the third PFET, the first NFET, the second NFET, and the third NFET are on the BOX layer.

10. A semiconductor structure, comprising: a substrate; a well in the substrate and biased differently than the substrate; a buried oxide (BOX) layer on the substrate; a first PFET having a first threshold voltage; a second PFET having a second threshold voltage less than the first threshold voltage; a third PFET having a third threshold voltage less than the second threshold voltage; a first NFET having a fourth threshold voltage; a second NFET having a fifth threshold voltage less than the fourth threshold voltage; and a third NFET having a sixth threshold voltage less than the fifth threshold voltage, wherein the first PFET, the second PFET, and the third PFET are on the BOX layer and over the well, and the first NFET, the second NFET, and the third NFET are on the BOX layer and over substrate, the well is an n-well that is biased to a power supply voltage; and the substrate is a p-type substrate that is biased to ground.

11. The structure of claim 10 , wherein: a transistor body of the first PFET and a transistor body of the second PFET each has a first dopant concentration; a transistor body of the third PFET has a second dopant concentration that is less than the first dopant concentration; a transistor body of the first NFET and a transistor body of the second NFET each has a third dopant concentration; and a transistor body of the third NFET has a fourth dopant concentration that is less than the third dopant concentration.

12. A semiconductor structure, comprising: a substrate; a well in the substrate and biased differently than the substrate; a buried oxide (BOX) layer on the substrate; a first PFET having a first threshold voltage; a second PFET having a second threshold voltage less than the first threshold voltage; a third PFET having a third threshold voltage less than the second threshold voltage; a first NFET having a fourth threshold voltage; a second NFET having a fifth threshold voltage less than the fourth threshold voltage; and a third NFET having a sixth threshold voltage less than the fifth threshold voltage, wherein the first PFET, the second PFET, and the third PFET are on the BOX layer and over the well, the first NFET, the second NFET, and the third NFET are on the BOX layer and over substrate, the second PFET is over a p-type back gate; and the second NFET is over an n-type back gate.

13. The structure of claim 12 , wherein: the p-type back gate is in the well; and the n-type back gate is in the substrate.

14. The structure of claim 13 , wherein: the substrate is a p-type substrate that is biased to ground; and the well is a single n-well in the p-type substrate and is biased to a power supply voltage using a single n-well contact opening.

15. The structure of claim 14 , wherein: the first PFET and the first NFET share a first common gate electrode; the second PFET and the second NFET share a second common gate electrode; and the third PFET and the third NFET share a third common gate electrode.

16. The structure of claim 10 , wherein: the substrate is a p-type substrate; the n-well is a single n-well in the p-type substrate and is biased to a power supply voltage using a single n-well contact opening; the second PFET is over a p-type back gate formed in the single n-well; and the second NFET is over an n-type back gate formed in the p-type substrate.

17. The structure of claim 16 , wherein: the p-type substrate directly contacts the BOX layer at a location directly underneath the first NFET; and the p-type substrate directly contacts the BOX layer at a location directly underneath the third NFET.

18. The structure of claim 8 , wherein: the single n-well directly contacts the BOX layer at a location directly underneath the first PFET; the single n-well directly contacts the BOX layer at a location directly underneath the third PFET; the p-type substrate directly contacts the BOX layer at a location directly underneath the first NFET; the p-type substrate directly contacts the BOX layer at a location directly underneath the third NFET; the second PFET is over a p-type back gate formed in the single n-well; and the second NFET is over an n-type back gate formed in the p-type substrate.

19. The structure of claim 1 , wherein: the first PFET and the first NFET share a first common gate electrode; the second PFET and the second NFET share a second common gate electrode; and the third PFET and the third NFET share a third common gate electrode.